People make a big deal about privacy, but the actual users of social media usually post stuff because they want everyone to know about it. The people who really care about privacy are not on social media. There's already a general understanding that when using the internet, you're getting tracked by various actors and people (even begrudgingly) accept that.
That same graph shows that Facebook usage has been dropping virtually every month before the election and after, totaling almost 50% loss. There's no exceptional change visible due to any particular event. There are some strong upticks in there as well, but the longterm trend is down.
I believe the most reasonable explanation is simply that people are moving on, just like they moved on from MySpace and other previously popular platforms. Young people are leading the charge here, it's just not as comfortable when your grandmother and high school teacher can see you on Facebook. It's uncool.